Nonuniformity-indicating parameters are measured using a tire uniformity inspection machine. In a typical tire uniformity inspection machine, tires are conveyed to a test station where each tire is mounted upon a chuck, inflated and rotated with its tread surface in forced radial contact with the circumferential surface of a loadwheel. The loadwheel is a rigid cylindrical structure which rotates freely on a spindle due to its engagement with the tire. To measure forces exerted on the loadwheel by the tire in directions of interest, the opposite ends of the loadwheel spindle are each fitted with force transducers. Those transducers typically comprise triaxial loadcell assemblies mounted for sensing forces in three mutually orthogonal directions, namely; the radial, lateral and tangential directions. For each of these directions, each loadcell assembly includes a series of four strain gauges connected in a conventional bridge arrangement so as to respond to forces acting in a particular direction by generating an analog signal whose instantaneous magnitude is correlated to the instantaneous magnitude of that force. The analog signals from the appropriate loadcells at opposite ends of the spindle are then combined in order to report the total force acting on the loadwheel in a particular direction. The combined force signal is then applied as an input to an analog signal processing network associated with one of a number of distinct measuring channels. Force measuring channels may be of either a "suppressed" or "unsuppressed" type.
A typical unsuppressed measuring channel has at its front end an instrumentation amplifier. In addition to amplifying the signal, the instrumentation amplifier serves to electrically isolate the force transducer as well as to add or subtract any required offset to the signal generated by the transducer. The output of the instrumentation amplifier is usually applied to an active filter to attenuate frequencies other than those of significance. The output of the filter represents an unsuppressed analog signal which is then applied to an analog to digital (A/D) converter and converted to a digital signal representing the nominal unsuppressed output of the channel. A computer samples the output of the A/D converter and digitally processes the information in order to calculate the value of a given nonuniformity-indicating parameter characterizing the tire under test. A plurality of different nonuniformity-indicating parameters characterizing the same tire are usually measured substantially simultaneously by each of the remaining channels of the tire uniformity inspection machine. Those parameters are then typically displayed, recorded and/or compared with specification criteria in order to initiate further action such as rejecting the tire if the specification criteria are not met or, in appropriate cases, initiating corrective measures such as grinding the tire in selected areas in order to improve its performance.
The average radial load impressed on the tire to establish a desired test condition is quite large in relation to the force variations actually generated by the tire. Thus, when measuring a nonuniformity-indicating parameter correlated to radial force variation, the total instantaneous force registered by the force transducer includes not only a component indicative of the force variations generated by the tire under test but also a much larger component representing the average radial load on the tire. During operation of a typical tire uniformity inspection machine for testing, the loadwheel exerts an average radial load of about two thousand pounds on the tire under test whereas the actual force variations generated by the tire are typically less than about twenty pounds. To increase the resolution of force variation measurements made under such conditions, it is common practice to measure such a parameter using what is known as a suppressed channel of the tire uniformity inspection machine in order to cancel the effects of the large average radial load. A suppressed channel is similar to the unsuppressed channel described above except that it includes a suppression network between the filter and A/D converter.
The suppression network includes a summer which subtracts from the unsuppressed signal, which represents the total force on the transducer, a suppression signal representing the average radial load on the tire in order to cancel that component from the unsuppressed signal. In order to minimize inaccuracies due to noise and quantization error, the resulting signal is then applied to an amplifier whose gain is selected to be large enough so that the magnitude of the largest signal expected to be generated by the amplifier will correspond to the upper end of the input range of the A/D converter. The output of the amplifier is applied to the A/D converter which, in response, generates a digital signal representing the nominal suppressed output of the channel.
After the output of the A/D converter associated with either a suppressed or unsuppressed channel is sampled by the computer, the computer derives an indicated value, I, for each data sample, A, by solving an equation of the form: EQU I=(A-T)*C EQUATION 1
In the above equation, the data sample term, A, corresponds to the "actual" output of the A/D converter of the channel as originally sampled by the computer when a tire is being inspected. The term T, for "tare", is a constant representing the magnitude of the observed output of the A/D converter with no external load applied to the force transducer. The term "C" is a calibration factor which the machine must apply so that the "indicated value" equals as nearly as possible the true externally applied force. The tare and calibration factor terms determined and stored prior to initiating out tire inspection operations. The calibration factor is determined by carrying out a calibration procedure.
That procedure includes applying a relatively small known force to the force transducer using weights of modest size. Such weights total less than about one hundred pounds and typically weigh about fifty pounds in all. The use of such small weights is possible because the strain gauge loadcells typically used as force transducers in tire uniformity inspection machines have a fairly linear transfer function (i.e., ratio of output voltage to applied force) over their more than two thousand (2000) pound typical operating range. Such weights are applied to the machine so as to act on the transducer along the same direction as the forces to which the channel being calibrated is to respond during tire testing. This is accomplished by attaching the weights to the loadwheel or to the loadwheel spindle using fixtures connected to the machine either directly or, indirectly through a cable and pulley system.
Determination of the calibration factor in the conventional manner was straightforward and was based solely on the magnitude of the digital signal generated by the A/D converter of the channel in response to the aforementioned weight. The calibration factor was calculated simply as the ratio represented by the magnitude of the output of the A/D converter to the magnitude of the known applied force.